The present invention relates generally to a cardiac telemetry system and, more particularly, to an FM demodulator, which may also be termed a frequency to voltage converter, for use in such a system.
Various systems have been proposed and developed for the transmission of information relating to coronary function to a remote point. The information related to coronary function may, for example, be an electrocardiogram (EKG) signal. Another example is a signal relating the functioning of an implanted cardiac pacemaker, for example the waveform or duration of a pulse generated by the implanted cardiac pacemaker.
Transmission of such information may, for example, be from an implanted device to a point just outside the patient's body. This may be accomplished by a radio frequency transmitter within the implanted device cooperating with a suitable external receiver, or, preferably, by an audio frequency oscillator exciting a coil functioning as the primary winding of a coupling transformer cooperating with an external pick up coil functioning as the coupling transformer secondary winding.
Another example of transmission of signals carrying information relating to coronary function is transmission to an entirely remote location, such as a hospital, via a radio or telephone communication link. In this instance, suitable sensing arrangements attached to the patient's body, which arrangement may be the inductive pick-up system mentioned just above, provide a suitable signal for transmission via the communication link to the remote location. At the remote location, the signal is suitably displayed, for example, by a strip chart recorder, and interpreted by qualified personnel who can then take appropriate action.
One particularly useful modulation technique in such systems is frequency modulation, particularly of an audio signal. For example, an audio carrier frequency of 1000 Hz might be employed, which is compatible with an ordinary telephone communication system. At the transmitting end of such a system, a suitable FM modulator, which may be termed a voltage to frequency converter, converts a signal waveform represented by a varying voltage into a signal of varying frequency. At the receiving end, a demodulator, which may be termed a frequency to voltage converter, is employed to reconstruct a replica of the original varying voltage waveform.
In such systems, it is desirable that highly effective frequency demodulators are employed. Two desirable qualities in a frequency demodulator are extremely rapid response to changes in input frequency so as to accurately reproduce the original signal waveform, and a broad input frequency range. Additionally, in order to avoid distortion in the replicated signal waveform, the demodulator should have a reasonably linear output voltage as a function of input frequency characteristic. Additionally, it is desirable that all of these objectives be met by a demodulator circuit of minimal complexity and cost.
However, typical signal waveforms encountered in cardiac telemetry systems place stringent demands on the elements of such systems, particularly on the demodulator elements thereof and particularly where an audio frequency carrier is selected and transmission is over a system of limited bandwidth, such as a telephone transmission channel. Typical signal waveforms include frequency components of only a few Hertz, as well as relatively high frequency components at the leading and trailing edges of pulse-like signal elements. In light of these requirements it is desirable to convey as much information as possible through an available channel. For in an audio frequency system having a carrier center frequency of 1000 Hz, it would be desirable to have a relatively broad input frequency range such that deviations of from -50% to +100%, or from 500 Hz to 2000 Hz, are accurately demodulated.
A number of demodulators have heretofore been employed or proposed for these purposes. Before proceeding with specific examples, it should be noted that there are few FM demodulators available, particularly in the audio frequency range, which have the desired broad input frequency range. A range of .+-.30% is much more typical than -50% to +100%. Additionally, a rapid response such that demodulator output voltage instantly follows changes in input frequency is unusual. Further, the problem of linearization is often not addressed in a cost effective manner. Considering some specific examples, in the cardiac function monitoring system disclosed in U.S. Pat. No. 3,885,552--Kennedy, a phase locked loop frequency demodulator is employed. In another approach, such as is exemplified by U.S. Pat. No. 3,099,800--Vinson et al, square wave pulses of varying duration depending upon the frequency of an incoming AC waveform are generated, and then integrated to provide a DC output. It will be appreciated that this typical approach in particular suffers the disadvantage, when employed in a cardiac signal telemetry system, of a lack of instantaneous response to input frequency changes. The output integration may occur over a period as long as ten cycles of the incoming waveform. One frequency demodulator of particular relevance with respect to certain aspects of the present invention is a video demodulator as disclosed in U.S. Pat. No. 3,614,641--Thompson et al. In the Thompson et al video demodulator, potentials are stored by capacitors in response to selected half periods of recorded FM signals, and whichever one of the stored potentials is indicative of the intelligence in the FM signal is selected for output. Of course there are other types of FM demodulator circuits, such as ratio detectors and discriminators, both of which employ resonant circuits tuned to a center frequency.
From these various prior art examples it can be seen that there remains a need for an FM demodulator especially suitable for use in combination with cardiac telemetry systems, and which effectively handles the signal waveforms found in such systems.